1. Field of the Invention
The present invention relates to a method and an apparatus for recording digital data. More particularly, the present invention relates to a method and an apparatus for recording digital data on a recording medium on an discrete basis in predetermined amounts.
2. Description of the Related Arts
Recording and reproducing apparatuses have been proposed which use a magneto-optical disc as a recording medium and record an analog audio signal on the magneto-optical disc by converting the analog signal into a digital audio signal. In such recording and reproducing apparatuses, digital data obtained by compressing a digital audio signal are continuously or discretely recorded on a magneto-optical disc in predetermined amounts.
Specifically, for recording, a digital audio signal obtained by performing digital conversion on an analog audio signal is divided into blocks in predetermined cycles and is subjected to data compression block by block (such a block is hereinafter referred to as a "sound group"). The compressed data are subjected to an encoding process for correcting errors and a modulation process for recording. The digital data which have been subjected to these processes are supplied to a magnetic head. A vertical magnetic field which has been modulated based on digital data supplied by the magnetic head is supplied to a magneto-optical disc to which an optical beam is directed by an optical head simultaneously to record the data thereon. The magneto-optical disc is formed with a recording track in the form of a spiral extending from the inner circumference to the outer circumference thereof along a groove which has been formed thereon in advance.
FIG. 1 shows the format of digital data which have been compressed from among signals recorded on a magneto-optical disc. The digital data are sequentially distributed to clusters C.sub.c, C.sub.c+1, C.sub.c+2, etc., each cluster being constituted by 36 sectors, i.e., sectors L2 through L4, S0 through S31 and L1.
The sectors S0 through S31 are data sectors which include main data such as digital audio data. The sectors L2 through L4 and L1 are referred to as "linking sectors" and are used for connecting a cluster C.sub.c to the preceding cluster C.sub.c-1 and the succeeding cluster C.sub.c+1. Among these linking sectors, the sectors L2 and L3 are sectors for run-in blocks; the sector L4 is a sector for sub-data (the contents of which are undefined); the sector L1 is a sector for run-out blocks. The size of each of these sectors is 2352 bytes and, therefore, one cluster contains 84672 bytes (=2352 bytes.times.36 sectors).
Recording on a magneto-optical disc is performed on a cluster basis, and it is recognized as described above that the clusters are discretely recorded on a magneto-optical disc. Therefore, a plurality of clusters which constitute continuous digital data are not necessarily physically continuous on a magneto-optical disc.
The detail of the sectors is shown in FIG. 2 wherein an area constituted by one sector, i.e., 2352 bytes, is illustrated in the form of a table which is constituted by 588 bytes and four bytes in the vertical and horizontal directions. In the following description, an area constituted by four bytes in the horizontal direction is treated as one address, and addresses 0 through 587 are assigned in the vertical direction. Therefore, the size of one address is four bytes.
Addresses 0 through 3 constitute a header, and the 12 bytes in addresses 0 through 2 are synchronization signals, labelled sector Sync, having a predetermined bit pattern. The first two bytes in address 3 represent the cluster number of the cluster to which this sector belongs (the cluster number of the cluster on the disc), labelled Cluster, and the next one byte represents the sector number of the sector, labelled Sector. The last one byte, labelled Mode, is assigned to data which indicates type of disc. For example, if an audio signal is recorded, Mode=2.
The 584 addresses from address 4 up to address 587 (an area of 2336 bytes) constitute a data area which contains audio data for 5.5 sound groups. Specifically, as also shown in FIG. 1, addresses up to address 4 of each sector serve as a header as described above. In an even-numbered sector, however, addresses from address 4 up to address 534 are grouped into five areas each containing 106 addresses. Those areas are treated as sound groups 0 through 4, respectively. The area constituted by the remaining 53 addresses from address 535 up to address 587 is treated as the first half of a sound group 5.
In the odd-numbered sector succeeding this even-numbered sector, an area constituted by 53 addresses from address 5 up to address 57, is treated as the second half of the sound group 5; addresses 58 through 587 are grouped into five areas each containing 106 addresses; and those groups are treated as sound groups 6 through 10, respectively.
The above-described formats of clusters, sectors and sound groups equally apply to a disc except that the contents of clusters are continuous in point of time but the physical positions of clusters on a magneto-optical disc can be discrete.
As described above, since it is recognized that digital audio data are discretely recorded on a magneto-optical disc on a cluster basis, it is necessary to manage the positions in the recording area of a magneto-optical disc wherein digital audio data are recorded. For this purpose, a management data area for recording management data which are used for managing the utilization of clusters is provided at the innermost circumferential portion of a magneto-optical disc.
Such a management data area is referred to as "TOC (Table of Contents)", and the above-described magneto-optical disc is provided with a "P-TOC" which cannot be rewritten by a user and a "U-TOC" which can be rewritten by a user. In addition to the cluster management table, the U-TOC records the name of the disc, the titles of pieces of music and date and time of recording where the audio data are music data. In the P-TOC, data or the like indicating the type of the disc are prerecorded by means of pits as physical irregularities.
Sector 0 which is the first sector of the cluster wherein the U-TOC is prepared is allotted to the cluster management table. FIG. 3 shows the structure of sector 0. The following example is a description of a configuration for recording of music information as recorded digital data.
Specifically, in the main data area, a pointer P-TNOn (n=1 through 255) related to the position where the n-th piece of music is recorded is written in each of the bytes from the second byte in address 12 up to the fourth byte in address 75. Start addresses and end addresses indicating the start and end positions of 255 parts of music are written in the first, second and third bytes in addresses 78 through 587.
In this case, the address pointed to by the pointer P-TNOn is the address wherein the start address of the n-th piece of music is written, and the succeeding address is the address wherein the end address is written. If P-TNOn=m, the start address of the n-th piece of music is written in an address (76+m.times.2), and the end address of the n-th piece of music is written in an address (76+m.times.2+1).
The clusters on the magneto-optical disc are used in a continuous manner from a position in which recording is started indicated by a start address up to a position in which recording is finished indicated by an end address. A continuous recording area specified by a pair of start and end addresses is hereinafter referred to as a "part". The number of such parts is 255 which is equal to the number of pairs of start and end addresses.
When clusters on the magneto-optical disc cannot be used in a continuous manner for a single piece of music, i.e., when a plurality of parts are used for a single piece of music, it is necessary to know the order in which those parts are used. A link pointer Link-P is prepared in the fourth address following each of the end addresses written in the addresses 78 through 587. A link pointer Link-P specifies the start address of the part succeeding the part to which the link pointer Link-P belongs as a part pointer P-TNOn does. If there is no succeeding part, i.e., the part is the last part of a piece of music, the link pointer Link-P is set to, for example, "0".
Mode data, labelled Track Mode, indicate the processing mode for each part and are written in the fourth part following each of the start addresses in the addresses 78 through 587. This makes it possible to identify whether or not a part is inhibited from being copied and written and whether emphasis is put on it or not.
The first two bytes of address 11 are a disc identification code labelled Disc ID. The next one byte is a pointer, labelled P-FRA, P-DFA indicating a defective area in the main data area. The last one byte is a pointer, labelled P-FRA, indicating the leading address of an area in the U-TOC wherein neither start address nor end address is written. Assume that a recordable area wherein digital data can be recorded is referred to as a "free area". Then, the first byte in address 12 is a pointer P-FRA indicating this free area. The pointers P-DFA and P-FRA point in the same manner as that of the pointer P-TNOn.
FIG. 4 shows the formats of a start address and an end address. Specifically, a start address and an end address are both data consisting of three bytes wherein the first 14 bits indicate the cluster number of the cluster on the optical disc; the next six bits indicate the sector number of a sector in the cluster; and the last four bits indicate the sound group number of a sound group in the sector. Therefore, the use of such start and end addresses makes it possible to manage and identify positions on a magneto-optical disc on a sound group basis.
The above-described formatting of digital data recorded on a magneto-optical disc allows additional recording of digital data based on music information, overwrite-recording and erasing of only one piece of music. Instead of actually erasing digital data in a part, the data in the part can be equivalently erased only by rewriting the data in the U-TOC to update the U-TOC so that it indicates that the part is a free area.
In a recording and reproducing apparatus as described above, when a plurality of parts Pi, Pj, Pk, etc. are physically continuously formed on a magneto-optical disc as shown in FIG. 5, non-recording areas C.sub.x each constituted by one cluster are provided between those parts. The length of one cluster corresponds to about three tracks (i.e., three turns of the disc) on average. As a result, even if the position of an optical beam for recording directed by the optical head is moved in the direction of the length or width of the recording track due to vibration or the like during recording, the position reached by the movement will be in a non-recording area C.sub.x. This protects the contents recorded in the adjacent positions against corruption.
However, recording involving the non-recording areas C.sub.x will result in a decrease in the areas among the recording area of a magneto-optical disk which can be actually used for recording due to an increase in the number of the non-recording areas C.sub.x if a single piece of music is recorded in a multiplicity of parts after repeated erasing and recording operations.
Further, as apparent from the above description on the U-TOC, the U-TOC does not directly manage the non-recording areas C.sub.x. For example, assume that the data recorded in the part Pj in FIG. 5 are erased to change the area wherein the part Pj has been recorded to a free area and, thereafter, other digital data are recorded in this free area (the former part Pj). Then, the first cluster of this free area (the former part Pj) is appointed to be a non-recording area C.sub.x, and new digital data are recorded starting with the cluster that follows the new non-recording area C.sub.x. As a result, the newly recorded part will be preceded by a non-recording area C.sub.x consisting of two clusters, one of which is the non-recording area of the former part Pj.
In addition, as a single piece of music is shared by an increasing number of parts as a result of repeated erasing and recording, the U-TOC becomes unable to manage all digital data recorded on the magneto-optical disc because the maximum number of parts which can be managed by the U-TOC is 255.
One possible solution to this problem is as follows. When a piece of music is erased, from among the parts which have been erased, recording areas whose lengths are equal to or longer than four clusters are recorded in the U-TOC as areas usable for recording, i.e., free areas.
However, if recording areas having lengths equal to or longer than four clusters are recorded in the U-TOC as free areas, there is a possibility that areas shorter than four clusters are produced after repeated erasing and recording. This results in a reducyion in the areas of the recording area of the magneto-optical disc which can be actually used for recording.
Further, some users repeatedly record data having a short duration corresponding to less than four clusters. In this case, even if such data are erased from the recording area of the magneto-optical disc, the erased area will not be managed by the U-TOC as a free area.
When new digital data are written over old digital data, i.e., when overwriting is performed, a non-recording area C.sub.x is formed as in the case of erasing as described above. However, the U-TOC does not directly manage non-recording areas C.sub.x. For example, if new digital data are written over the area of the part Pj in FIG. 5, the first cluster of the part Pj is appointed as a non-recording area C.sub.x as in the case of erasing, and the new digital data are overwritten starting with the cluster that follows this new non-recording area C.sub.x. As a result, the newly formed part will be preceded by a non-recording area C.sub.x having a size corresponding to two clusters. Therefore, repeated overwriting operations will also result in a decrease in areas of the recording area of the optical disc which can be actually used for recording.
In order to prevent areas of a magneto-optical disc which can be used for recording from being decreased, it has been proposed to perform processing as described below during erasing. Assume that the part Pj in FIG. 5 is to be erased during an erasing operation performed in the above-described apparatus Then, the part Pj is managed on the U-TOC as a free area. The non-recording areas C.sub.x provided before and after the part Pj are also regarded as free areas and are managed on the U-TOC. In other words, when TOC data in the U-TOC are updated to treat the part Pj as a free area, the area from the non-recording area C.sub.x preceding the part Pj up to the non-recording area C.sub.x succeeding the part Pj is now regarded as the part Pj and is managed on the U-TOC as a free area.
In the case of the overwriting of digital data, the non-recording area C.sub.x preceding the part Pj on which overwriting is to be performed is similarly recorded as the part Pj, and new digital data are written over the area wherein old digital data are recorded and the non-recording area.
In this way, the non-recording areas C.sub.x are included in the free area when erasing is performed. This makes it possible to prevent the areas in the recording area of a magneto-optical disc which can be used for recording from being decreased. In the case of the overwriting of new digital data, there will be no decrease in recordable areas for the same reason.
As apparent from above, however, the U-TOC does not directly manage non-recording areas C.sub.x. Therefore, to register the part Pj and the non-recording areas C.sub.x before and after the part Pj as a free area, it is necessary to search all the start and end addresses registered in the U-TOC for an end address AEi which is the nearest end address preceding the start address ASj of the part Pj and for a start address ASk which is the nearest start address succeeding the end address AEj of the part Pj.
Thus, in erasing a piece of music recorded using many parts or many pieces of music, many parts are erased, and necessary addresses must be found for all of such parts from among all the start and end addresses registered in the U-TOC.
This results in a significant increase in the time required for the erasing of digital data.